Pigment paste resins are an important component of cathodically depositable coating compositions and exert substantial influence on the bath parameters and on physical and chemical characteristics of the paint films.
Pigment paste resins based on epoxy resins have been referred to in the patent literature over a long period of time. Thus, U.S. Pat. Nos. 3,936,405 and 4,035,275 disclose cationic, water-thinnable polymers which are obtained by the reaction of polymers containing 1,2-epoxide groups, especially of polyglycol-modified epoxy resins, with amine salts, phosphine-acid or sulphide-acid mixtures giving rise to onium groups. Similar products which are cross-linked by transurethanization are disclosed in DE-PS 2,634,211 C2; DE-OS 2,634,229 A1, or DE-OS 2,928,769 A1.
Pigment wetting of resins crosslinkable by transesterification can be enhanced by the introduction of quaternary ammonium groups in the resins, making their use as pigment grinding resins possible. This variant is disclosed, for example, in DE-OS 3,322,781 Al. To improve the stability of these pigment pastes, DE-OS 3,622,710 A1 suggests formation of quaternary ammonium groups in the presence of the pigment powder. Pigment paste resins which yield coatings with improved corrosion resistance are obtained, according to EP 107,088 A1, by reacting monoepoxides, such as alkylene oxides, glycidyl esters, or glycidyl ethers with long-chain tertiary amines in the presence of an acid and/or water with the formation of quaternary ammonium groups. According to EP 107,098 A1, higher pigment loading is possible if these systems additionally contain amide groups. According to EP 107,089 B1, polyepoxides can be reacted in the same manner, making it possible to use higher proportions of pigments. Pigment paste resins based on alkylphenol-modified epoxy resins containing sulphonium groups, such as those disclosed in EP 251,772 A2, are alleged to permit formation of thicker films. In another type of a pigment paste resin disclosed in EP 199,473 A1, an epoxidized polybutadiene is reacted with a secondary amine. The addition of an acid and a monoepoxide leads to the introduction of quaternary ammonium groups.
Pigment paste resins based on epoxy resins comprising oxazolidine groups are disclosed, for example, in AT-PS 365,214 and AT-PS 380,264. Resins of this type exhibit good general characteristics necessary for pigment paste resins, and have found use in practice despite drawbacks connected with their dispersibility in the bath material.
The voluminous literature relating to the formulation of pigment pastes for cathodically depositable coating compositions emphasizes that there is a problem in this field for which there is still no completely satisfactory solution despite research extending over a number of years and despite numerous suggestions for solving the problem.